I Solemnly Swear
by OfTheFullMoon
Summary: No group of pranksters would be without a way of making sure they don't get caught, and what better way to avoid detection than to create a certain magical map?


_Characters belong to J.K. Rowling._

* * *

><p>"You know," fifteen-year-old James Potter began. His three best friends looked up from where they were sitting near the fireplace in the Gryffindor common room. Sirius was sprawled out on one of the couches, Remus was lying on the thick red rug with a book in front of him, and Peter was sitting cross-legged near Remus, while James was lying in a large chair, his legs hanging over one of the chair's arms.<p>

James' friends looked at him expectantly. "If we're going to continue to establish a name for ourselves in Hogwarts, being the perfect pranksters that we are, we're going to need to find better ways to carry out our schemes without getting caught. I for one spent too many precious hours of these past four years in detention. Think of all the things I could have done with that time."

"True." Sirius added. "Detention took up most of our weekends, didn't it? We could have been exploring Hogwarts, preparing more elaborate pranks, sneaking food from the kitchen…"

"Studying for your exams, thinking of ways to _keep_ your house from losing points, or just taking some time to settle down and be quiet for longer than thirty seconds." Remus finished.

"And ruin our reputation?" Sirius asked indignantly. "Come on, Rem. With our tricks, the four of us will be famous for the rest of Hogwart's existence by the time we get out of here."

"Make it the _three_ of you, and I'll consider my years here well spent." Remus turned a page of his book, chin propped up on one hand.

"And what would we do without you?" James asked. "You, our brilliant planner, who can think of every possible thing that would go wrong and inform us how to avoid it? We'd spend every day in detention if it wasn't for you."

"True." Remus agreed. "Although simply weekends are a lot less than you deserve."

"But unfortunately, every weekend is still dozens of weekends too many. So that's why we need to find a way to be even more hidden."

"But we've got your dad's invisibility cloak," Peter said slowly. "I don't think we can be much more hidden than invisible."

"Yeah, but it keeps getting harder for all four of us to fit under that cloak. We're not exactly as small as we were our first year." Sirius pointed out. "James is right. If we're going to keep this up for the next few years, we've got to get better. By the time we're outta here we need to be undetectable – invincible when it comes to pranking. You're the smart one, Rem. Is there any sort of spell that could keep us from begin caught?"

"Something like, I dunno…helping us be in two places at once, or making everyone believe us when we say we're innocent, or making sure no one's within a mile of us when we pull our pranks?" James asked hopefully.

Remus sighed and looked up at his eager friends with careful patience. "If you'd been paying attention in any of the classes we've had in the past four years, you'd know that you can only be in two places at once by using a time-turner, and there's no way a bunch of fifth years are going to get their hands on one of those. Plus, it's way too risky to use for something as trivial as pranks. You could probably use the Confundus charm on anyone who saw you to make them forget they saw you, but I think that would get you in some serious trouble. There's Homenum Revelio, which would reveal any human presence nearby. That might work."

"Hmmm." Sirius looked thoughtful. "Would we be able to detect the ghosts or Filch's nightmare of a furball?"

"It wouldn't work on the cat, and I doubt it would work on the ghosts. Granted, they were once living humans, but I doubt that counts since they're now dead."

"What if we could use that spell on some sort of object that would detect people for us? Something we could always have with us? That way we wouldn't have to cast the charm every time we turned a corner." James sat up in his chair and looked around at his friends for their opinions.

"You mean like a quill or a book or something?" Peter asked. "Something that wouldn't look suspicious?"

"Exactly, Pete!" James exclaimed. "What do you think, Rem? Would it work?"

"I suppose it's possible." Remus answered thoughtfully. "I've never actually tried it myself. Although I'm guessing that that sort of magic is way beyond us right now."

"You never know until you try." Sirius grinned.

Remus sighed and shut his book, looking up at the matching pleading puppy dog expressions on his two dark-haired friends. "Fine. I'll help you. You probably wouldn't stop nagging me until I did anyway."

"Nope." James affirmed. "Anyway, you know you love getting into trouble with us." He grinned at Remus' incredulous stare. "You're just too good to admit it." As he ducked the book that was thrown his way, he didn't miss the half-hidden smile from the quieter boy.

"All right, mates." Sirius said. "Tomorrow is our first weekend as fifth years at Hogwarts. I never thought I'd say this, but we'll start the year off with a bang…in the library. Researching precious information to create the most magnificent device to aid the most infamous pranksters in the history of the school! The first and last weekend worth sitting in a room full of dusty old books."

Remus snorted. "We'll be lucky if it takes us half a year of weekends sitting in the library. It's not going to be something we can find a quick charm for. We don't even know _what_ object we're enchanting!" He pushed himself into a sitting position and began to count off the list of what they would have to do. "So task number one: decide what we're enchanting. That will make it easier to find the right spells. Two: find the spells that will actually work on said object _and_ that a group of fifth years will actually be able to cast. Three: enchant the chosen object. Four: make sure it actually works. If not, then it's back to step one."

Peter looked depressed at the idea of the large amount of work ahead of them. James and Sirius merely looked at each other and grinned.

"Always the optimist when it comes to pranking, aren't you?" James asked cheerfully.

"Well someone has to keep their head on straight."

"So. Library tomorrow?" Sirius asked. "After breakfast?" At the three affirming nods he received, Sirius broke into a huge grin radiating excitement plus a bit of mischief. "Perfect."

* * *

><p>Despite the group's enthusiasm of the previous night, Remus practically had to drag his friends out of bed to go to breakfast at eight o'clock.<p>

"It's too _early_!" James protested. "And it's a _Saturday_! What person in his right mind would dare open his eyes before at least ten o' clock?"

Remus rolled his eyes and began the task of leading a still yawning Peter out from his nest of blankets and shoving him in the direction of their bathroom. "Stop whining and get out of bed. This was your idea."

"It's one I'm regretting." James fumbled with his sheets, attempting to untangle himself as Remus threw back the curtains around Sirius' bed.

After shoving his glasses onto his face, James glanced over at Remus to discover him unsuccessfully trying to pull Sirius out of bed. He rolled his eyes and dragged himself out of his own bed to go help his friend wake their unconscious comrade.

"Sirius!" James yelled, giving his best friend a forceful shove. "Move! Spending the day in the library was your suggestion, so get up and take the lead in our new adventure!"

Sirius cracked open one eye and glared at his two friends, making no effort to move.

Remus was nearing the end of his patience. "You drag yourself out of that bed or Merlin help me, I'll jinx your sorry self all the way down to the Great Hall!"

That got the boy's attention real fast. "You wouldn't!"

The werewolf leaned in closer to Sirius, his hazel eyes snapping. "Watch me."

Sirius sent a pleading look towards James, who simply held up his hands defensively.

"Fine." Sirius muttered, climbing out of bed. "Thanks, James."

"Anytime, mate."

Remus looked extremely pleased with himself at managing to get his three friends out of bed early on a Saturday morning.

"Now if you three think you can keep yourselves moving, I'll wait for you in the common room and then we can grab something to eat before we hit the books."

James nodded while Sirius shrugged. As their friend headed downstairs, Sirius cast one final look towards his warm bed, to which James shook his head warningly.

"You know he'll murder you if you so much as think about skipping out to sleep."

Sirius grimaced. "For such a quiet and patient person, he really can be a little scary sometimes."

The three friends managed to get themselves together in the next twenty minutes, finding Remus curled up in a chair in the common room, poring over a book. He glanced up at their arrival, and slid the book into his bag.

"Ready?"

"Absolutely." James answered for the group.

The four made their way down to the Great Hall to join the handful of other students who actually bothered to wake up early on a Saturday morning to eat breakfast. Once the sleepier three of the group started eating, they began to perk up more and got excited over their new project again.

On their way to the library, they started discussing their ideas for what they were going to enchant.

"A magnet?" Peter offered. "It could float in front of us and hover in the direction of anyone nearby."

Remus shook his head. "It would be a problem if there was more than one person, and we couldn't walk around with a swarm of magnets following us. _That_ wouldn't look suspicious at all."

"A compass?" James suggested.

"Similar problem." Remus pointed out. "It can only spin in one direction at a time. How about a book? When it detects people nearby, their names will appear on the page and how close they are to us."

"Maybe." Sirius said. "But if they're moving, the book will have to write something new every time they walk another few feet. We'd be so busy trying to keep up reading that we'd get caught. What about a picture? The faces of nearby people would show up in the photo."

"That wouldn't tell us where they are, though." James sighed.

By that point they had reached the library doors. "Well," Remus said thoughtfully. "We'll come up with something eventually. Right now we need to work on finding all the detection spells that we can. Once we've got a pretty good list, we can start to map out the rest of our plan."

James froze suddenly, a look of revelation on his face. "Merlin's beard, Remus. You. Are. Brilliant." The other three looked at him blankly. "You said "map!" _Map!_ We'll create a map of Hogwarts that will tell us where everyone is!"

"A map of the whole castle?" Remus asked incredulously.

"Why not? Can it be done?" James asked.

"I don't know. That's going to take a lot of work. Do you realize that we somehow have to map out _everything_ in the school? Every last hallway and forgotten room if we want to have the whole place show up accurately? And somehow get the map to recognize every living person who so much as sets foot in Hogwarts?"

"I dunno, mate." Sirius said thoughtfully. "It's sounding pretty brilliant to me. It would be completely thorough in checking for people, and we'd never get lost. I say we give it a try."

Remus looked at Peter, who was nodding enthusiastically in support of James and Sirius. Remus shrugged, running the idea through his head. "If we can make it work, it _would_ be completely brilliant. We may as well see what we can come up with."

The group claimed a table in the very back of the library, as far away from prying eyes as they could get.

"All right," Remus said quietly. "Spread out and find any books on spells you can, especially those that mention anything about any form of detection."

"Any books on spells?" Sirius muttered to James. "That's the whole bloody library!"

Remus looked at him pointedly. "The sooner we start gathering resources, the sooner we can start reading. We'll make a list of any potential spells, and from there, we'll see what we can do."

Within fifteen minutes, the group had returned to the table, arms piled high with books. To each member's credit, they put as much effort as possible into finding spells. Even James and Sirius, who tended to slack off when it came to research, dove into the books to find the best information they could for their biggest project yet. They worked constantly throughout the day, stopping only for meals, and by the time Madam Pince came to shoo them out of the library so she could lock up, all four wizards found themselves with headaches from going through so many books.

* * *

><p>"So what have we come up with so far?" James asked from where he had thrown himself across his bed.<p>

Remus studied his list of notes. "We've got quite a few spells. I think the most promising ones that we have so far are Homenum Revelio, a stealth sensoring charm, and a supersensory charm. If we can take Homenum Revelio and find a way to make it reveal all human presences, instead of just those nearby, that will be the base spell for the map. The stealth sensoring charm will detect anyone under a magical disguise, so that will allow the map to sense any student who got him or herself an invisibility cloak or the like. If we add the supersensory charm on top of that and somehow get it to apply to the map instead of the caster, we can give the map the ability to have enhanced senses, which could solve the limited range of Homenum Revelio."

"That's a good number of 'if's.'" James said doubtfully. "Is it even possible to twist those spells around?"

"I don't know." Remus answered, gazing thoughtfully at the parchment in his hand. "If we can't, we'll find something else to use." He returned the paper to his book bag and looked around at his friends. "So, I was thinking that instead of having all of us research tomorrow, why don't we split up? I'll research more spells, while the rest of you can either start taking measurements of the castle or help me find spells."

The other three looked at each other, then James spoke up. "Sirius and I will start measuring."

Sirius nodded in agreement with James, and Peter looked uncertainly between the two groups before speaking. "I'll help Remus research. Maybe I can also find some sort of blueprints for the castle or something."

"That's a good idea." James said. "If we could find something like that, it could save us a lot of time."

"That's settled, then." Sirius said. "Let's all meet tomorrow night and see what we've got."

* * *

><p>Sunday passed in a flurry of activity for the young wizards as they worked at their assigned tasks. At the end of the day they gathered in their room for the second night to compare notes.<p>

"James and I got a good bit of the first floor done, but it looks like it's going to take us longer than we thought. Who knew that there were so many rooms on one floor?"

"It also takes forever to get everything measured correctly." James added. "So how did you two make out?"

"Peter came up with a brilliant idea." Remus said, looking at their friend who blushed and shrugged. "He suggested that we find a way to make the map only respond to us, that way no one else will be able to use it."

"Perfect!" James said. "How will we do that?"

"We thought about it and decided that we would disguise the map. It would look like a plain piece of parchment until we use a password to make the map appear. Once we're done using it, we can use a closing password to wipe the map and make the paper look blank again." Smiling at the incredulously delighted looks on James and Sirius' faces, Remus continued. "I was thinking that we could use either Aparecium or Specialis Revelio to open the map, and either Deletrius or Finite could possibly work to erase the map."

"You two are brilliant." James said admiringly. "So I suppose until we get the castle mapped and all of the spells decided on, then there's not much else we can do."

"That's our two main tasks right now." Remus agreed. "Hopefully it won't take us too long."

Despite the group's hard work, the Christmas holidays arrived, and the snowy days found the four sitting in their empty common room, having chosen to stay and work on their map in the quiet of the castle.

"You know," James said. "We should have code names."

"Why?" Remus asked, looking at James with an arched brow.

"For fun. And to keep our identities a secret in case someone ever steals the map from us. It'll make us more mysterious."

"If we have individual code names, we should have a group name, too." Sirius added. "We can put that on our map, so that it'll have some sort of impressive name instead of just 'Map of Hogwarts.'"

"Alright, so what should our names be?" James asked, shifting his position in the armchair he was sitting in to face Sirius.

"They should have something to do with our animagi forms." Sirius said, referring to their latest achievement of that year.

"Right. You should be…" James squinted at Sirius, wracking his brain for something clever. "Hound?"

"No." Sirius said flatly.

"Midnight?" Peter offered. "Your fur is really dark."

"Maybe." Sirius said thoughtfully.

"How about Blackie?" Remus said dryly, not looking up from the parchment he was writing on.

Sirius shot his friend a glare. "That's not even funny."

"Padfoot?" James offered.

"What on earth is a Padfoot?" Sirius asked.

"Well, a dog has padded feet, right? And you're so quiet in your animagus form, it's hard for us to hear you. So…" James looked at his best friend expectantly.

"Padfoot." Sirius cocked his head thoughtfully. "Not bad. Your turn, James. How about Buck?"

"Merlin, no!" James laughed. "How about Stag instead?"

Remus rolled his eyes, trying to smother a smile.

"Antlers?" Peter suggested.

"Unusual, but interesting. But how about Prongs instead?" Sirius asked. "It's quirky and unforgettable, just like you."

"All right, Prongs it is! Remus next."

"Wolfie." Sirius offered promptly.

Remus pointed his quill threateningly at Sirius as the rest of them burst out laughing.

"Howler?" James said, still laughing.

"Not better."

"Moon." Peter said.

"How about Moony?" Sirius offered.

"That makes me sound like I'm absent-minded." Remus complained.

"Come on, we know you're anything but." James said. "Besides, it's got an innocence about it. Fits you perfectly."

"Peter's next!" Sirius said, not giving Remus a chance to protest any longer.

"Mouse." James said.

"No! I'm a _rat_, not a mouse!" Peter looked greatly offended at the name.

"Scamper?" Remus offered. "He's really fast at darting around."

"Nah, how about Longtail?" Sirius suggested.

"Wormtail!" James said jubilantly. "'Cause that's what your tail looks like, Pete! A long skinny worm!"

Peter looked uncertain, but shrugged. "I feel like that's a stupid name."

"They're all stupid names." James agreed cheerfully. "And that's why they're so brilliant. Anyone who heard us using them would never know what we're talking about! They're secret references to our animagi forms that no one but us could understand! We're brilliant!"

"Or mental." Came the comment from Remus.

"What about our group name?" Peter reminded them.

"The Pranksters!" James beamed.

"The Mappers." Peter said.

"HF's." Sirius said dramatically. "Hogwart's Finest."

They spent a few minutes laughing over that one, finally pulling themselves together enough to continue.

"The Marauders."

The other three looked at Remus, turning over the name in their minds.

"What's a marauder?" Peter asked hesitantly.

"It's another word for a raider or a pillager. With all the sneaking around and illegal stuff we do, I figure it fits."

"Of course it does!" James spouted. "The Marauders – controlling Hogwarts with the Marauder's map."

"I think you've just named our map there, Prongs." Sirius said with a grin.

"Why thank you, Padfoot. My brilliance strikes again." His comment was marked with a groan from the other three before they continued their discussion on their map.

* * *

><p>It was nearing the end of the school year when the Marauders finally huddled together one Saturday night to finally complete the creation of what would hopefully be their greatest achievement.<p>

"All right," Remus said, laying out all their information on the floor of their room. We have every inch of this castle mapped down to the last hidden passageway and our spells chosen. The only thing left is to decide on our passwords. What should we use to open the map?"

""Open Sesame."" Sirius joked.

"That one was taken a long time ago." James teased. "What about "Reveal your secrets?""

"Lame." Sirius frowned. "It needs to be something no one else would think of."

"What about something like "I need the map to pull pranks?"" Peter suggested.

"That's a good start." James frowned and looked at the parchment in front of them. "How about, "I swear I will only use this map for pranking?""

""I swear" is good." Remus added. "How about "I solemnly swear I will only use this map for pranking?""

"It's a bit of a mouthful, isn't it?" Sirius asked. "If we're in a hurry, then "I solemnly swear I will only use this map for pranking" could be a little hard to get out quickly."

""I solemnly swear…"" James looked thoughtful as he ran through different ideas. ""I'm up to no good!" How's that sound?"

Each wizard thought for a second, then nodded.

"And for closing the map?" Remus asked.

""Mischief managed."" Sirius said instantly, and was met with agreement from his friends.

"Alright," Remus said, taking a deep breath as he adjusted the parchment in front of them. "Let's begin."

The four wizards knelt around the map, wands in hand. Each began speaking at the same time, murmuring spells as the parchment began to shiver in place from all the magic directed at it, its ink inscriptions fading in and out and twisting across its surface as it adjusted to each new bit of magic.

When they had finished, each Marauder sat back, looking at the blank parchment with a nervous excitement.

"Did it work?" Peter asked anxiously.

"Let's find out." Remus said, pointing his wand at the map. "I solemnly swear that I am up to no good."

Ink began to curl across the paper, forming words in graceful script.

"_Messrs. Moony, Wormtail, Padfoot, and Prongs, Purveyors of Aids to Magical Mischief-Makers, are proud to present the Marauder's Map…"_


End file.
